The intent of the proposed study is to elucidate the factors important to the pathogenesis and epizootiology of Marek's disease. It has been determined that a group B herpesvirus, if not the sole causative agent, is at least fundamental to the etiology of this lymphoid neoplasm of birds. Nearly all flocks become and remain infected throughout their lives. Yet, the incidence of neoplasms varies from negligible to greater than 50% of the flock. Therefore, we desire to identify the influential factors and determine the mechanism(s) by which they are operative. In the course of these studies we will examine the role of immunity and immune mechanisms, genetic constitution, viral interference, age factors, mode of transmission, and the pathogenicity and immunogenicity of viral strains as they affect the outcome of a virus-host encounter. The development of purified virus strains and virus-free strains of chickens will be fundamental to these studies.